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Re: Timex Sinclair 2068 faults

Posted: Wed Nov 28, 2018 3:06 am
by gammaray
I noticed the original is better resolution. I shouldn't have placed inline. Maybe this one is better. Remove ".txt"

Re: Timex Sinclair 2068 faults

Posted: Wed Nov 28, 2018 3:41 am
by dessony
https://8bit-museum.de/scans/timex/ts2068schematic.jpg

Please check an U1, the label should read "A78S40", not "A78840". Blame a person who clean the schematic up!

Re: Timex Sinclair 2068 faults

Posted: Wed Nov 28, 2018 12:25 pm
by 1024MAK
Anyway after all the schematic shenanigans... back to the subject of a suitable PSU.

The PSU can be any DC type that is rated at 1A (or greater) with an output voltage of 15V DC to 17.5V DC. This would be for an unregulated type.
For a regulated type, the voltage range is slightly greater, 14.5V DC to 22V DC. A DC PSU is only a regulated type if it says it’s regulated on the label.

Of course, the polarity has to be correct unless you like magic smoke....

Mark

Re: Timex Sinclair 2068 faults

Posted: Wed Nov 28, 2018 3:47 pm
by gammaray
So are regulated the same as switching? If not are the methods of controlling voltage the same?

Re: Timex Sinclair 2068 faults

Posted: Wed Nov 28, 2018 6:43 pm
by 1024MAK
gammaray wrote: Wed Nov 28, 2018 3:47 pm So are regulated the same as switching? If not are the methods of controlling voltage the same?
A good quality switching type (from now on I will use the acronym SMPSU for switch mode power supply unit) will most probably be regulated. These have a feedback system to sense the actual voltage at the output terminals. So that when the current required by the load (the device connected to it, like a computer) increases, or decreases, it can sense this and compensate. This ‘feedback’ is so quick that even with a multimeter on the output terminals, you should not see the voltage change. A good quality SMPSU if bought from a trusted load retailer (meaning in your own country) should also be safe and meet any requirements by the laws and regulations in your country.

Cheaper types of SMPSU are semi-regulated (for want of a better term). They don’t have the same level of ripple that an unregulated conventional (heavy because there is a 50Hz or 60Hz mains transformer inside) PSU suffers from. But the output voltage will drop as the current required by the load increases. This is because there is no sensing of the voltage at the output terminals. The control circuitry is all on the primary (mains) side.

Watch out for really cheap SMPSU. Especially those built to a price point. For example, for sale on eBay from China at low prices. Never mind that their performance is crap, the manufacturers think that the isolation gap between the mains side and the output side can be fractions of a mm wide ( 1/32 of an inch). Or that a single layer of thin tape is fine. And they mostly lack any radio interference components or any fuses.

In practice, if the cheaper design of SMPSU are safe (bought from a trusted load retailer), then for suppling a device which was originally powered by an unregulated conventional PSU, these units are fine.

I hope this helps :mrgreen:

Mark

Re: Timex Sinclair 2068 faults

Posted: Thu Nov 29, 2018 7:10 am
by dessony
Gammaray,

Your TS2068 MIGHT have some bad capacitors. I also want to thank you very much for uploading the TS2068 technical book PDF file here yesterday or two days ago. Check these electrolytic capacitors numbers : c3 / c21 / c41 / c40 / c36/ c58 / c67 / c5 and c18. And a tantalum capacitor number is c71.

DesSony

Re: Timex Sinclair 2068 faults

Posted: Thu Nov 29, 2018 9:31 am
by McKlaud
Having some experience with TS2068 I would also recommend a full recap of any electrolytic caps. With my TS2068 I was fighting with wash out colours and after recap they were back to NTSC normal. I would not worry about tantalum one as this type can last forever.

Re: Timex Sinclair 2068 faults

Posted: Thu Nov 29, 2018 9:51 am
by 1024MAK
McKlaud wrote: Thu Nov 29, 2018 9:31 am I would not worry about tantalum one as this type can last forever.
Tantalum types degrade in a different way. If a suitable voltage rated type has not been used, switch on cycles and voltage spikes will cause internal damage. Then they sometimes go short circuit. If the available supply current is high, they sometimes go BANG!

If however a suitable voltage rated type has been fitted, and the power on surge current is limited, then they can last a very long time.

Note that there are various criteria where the voltage rating should be derated. So typically in a circuit, their voltage rating should be triple the actual supply voltage. So on a 5V rail, use parts rated at 16V. For a 12V rail, use parts rated at 35V.

Mark

Re: Timex Sinclair 2068 faults

Posted: Thu Nov 29, 2018 4:38 pm
by gammaray
Replacing caps' ... more practice on my soldering. Got a new tin of flux.

It's gonna get ugly... got some wick too!

Re: Timex Sinclair 2068 faults

Posted: Thu Nov 29, 2018 4:57 pm
by Lardo Boffin
Don’t worry! I was a soldering noob before fitting my first ZX8-CCB and have progressed to being slightly less of a noob completely rebuilding a ZX81 (albeit rather slowly thanks to a duff 6.5 resonator) and having refurbished loads of Sinclairs. Rubber key Spectrums are good practice - plenty of caps.

As Mark says - practice, practice, practice!